Wearable sweat monitoring represents an attractive opportunity for personalized healthcare and for evaluating sports performance. One of the limitations with such monitoring, however, is water layer formation upon cycling of ion-selective sensors, leading to degraded sensitivity and long-term instability. Our report is the first to use chemical vapor deposition-grown, three-dimensional, graphene-based, gradient porous electrodes to minimize such water layer formation. The proposed design reduces the ion diffusion path within the polymeric ion-selective membrane and enhances the electroactive surface for highly sensitive, real-time detection of Na+ ions in human sweat with high selectivity. We obtained a 7-fold enhancement in electroactive surface against 2D electrodes (e.g., carbon, gold), yielding a sensitivity of 65.1 ± 0.25 mV decade-1 (n = 3, RSD = 0.39%), the highest to date for wearable Na+ sweat sensors. The on-body sweat sensing performance is comparable to that of ICP-MS, suggesting its feasibility for health evaluation through sweat.
Keywords: chemical vapor deposition; electrochemistry; graphene; ion-selective electrodes; sweat sensing; wearable electronics.